1. Field of the Invention
The present invention relates to an espresso coffee machine with a brewing unit.
2. Description of the Related Art
A prior art espresso coffee machine of this kind includes a cylindrical brewing head having a sieve insert with a bottom in which an outflow opening is provided (DE 30 35 157 A1). The outflow opening forms a valve seat for a check valve, or pressure valve, including a spring carrying a valve cone engaging in the outflow opening of the sieve insert. When preparing a coffee beverage by means of such an espresso machine, the check valve is first closed by the spring whereby the pressure in the brewing head is increased as a consequence of the inflowing brewing water until the check valve opens and the coffee beverage may flow through the coffee outflow into coffee cups while on the surface of the coffee beverage, the espresso, the desired coffee foam is being formed which is referred to as the crema. The check valve is, therefore, also referred to as the crema valve.
In most case, crema valves of this sort include a spring-loaded ball or a ball end arrangement and are suited to build up in the brewing chamber a pressure of at least about 5 bar.
While an espresso-typical crema constitutes a quality feature of a good espresso, there are kinds of coffee beverages which can be prepared by the same espresso coffee machine as the espresso, where a crema is not always desired. This is particularly so in the case of a simple coffee also referred to as morning coffee which is brewed in larger portions than an espresso. A further known kind of coffee beverage referred to as “Schümli” coffee, on the other hand, ideally provides for a floating foam which is characteristic of it.
An espresso coffee machine has already been known by means of which various kinds of coffee beverages as selected by the user may be prepared and which, to this end, includes a valve device shaped as a control valve device between the brewing chamber and a beverage outlet while means are provided to be controlled by the user for varying the amount of the coffee beverage passed per time unit through the valve device (EP 1 133 944 B1). To this end, the valve device includes a spring-loaded valve body movable relative to a valve seat wherein the spring endeavors to keep the valve body in an open position and wherein the valve body is moved by the coffee beverage under pressure against the initial stressing force of the spring into the locking position. As means for varying the amount of the coffee beverage passing per time unit through the valve device, means for varying the initial stressing force of the spring might be suited, and/or the outlet cross section flow-technically relevant of a valve chamber of the valve device may be variable. Such means for varying the amount of coffee beverage passing per time unit through a control valve device, however, have to be manufactured rather precisely particularly if the outlet cross section of the control valve device should be varied. A long-term and constant effect is questionable considering that the flow rate of the brewing water flowing through the coffee powder cannot be kept by the control device on an absolutely constant value because the control behavior of the valve device, inter alia, depends on the viscosity of the passing liquid (column 3, line 56 through column 4, line 9). It is also of disadvantage that the control valve device between the brewing unit and the beverage outlet requires a sufficient installation volume within the espresso coffee machine.